gcc12, gcc13: always mangle __FILE__ into invalid store path

Without the change `__FILE__` used in static inline functions in headers
embed paths to header files into executable images. For local headers
it's not a problem, but for headers in `/nix/store` this causes `-dev`
inputs to be retained in runtime closure.

Typical examples are `nix` -> `nlonhmann_json` and `pipewire` ->
`lttng-ust.dev`.

Ideally we would like to use `-fmacro-prefix-map=` feature of `gcc` as:

  -fmacro-prefix-map=/nix/store/$hash1-nlohmann-json-ver=/nix/store/eeee.eee-nlohmann-json-ver
  -fmacro-prefix-map=/nix/...

In practice it quickly exhausts argument lengtth limit due to `gcc`
deficiency: https://gcc.gnu.org/PR111527

Until it;s fixed let's hardcode header mangling if $NIX_STORE variable
is present in the environment.

Tested as:

    $ printf "# 0 \"/nix/store/01234567890123456789012345678901-pppppp-vvvvvvv\" \nconst char * f(void) { return __FILE__; }" | NIX_STORE=/nix/store ./gcc/xgcc -Bgcc -x c - -S -o -
    ...
    .string "/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-pppppp-vvvvvvv"
    ...

Mangled successfully.

0 files changed, 0 insertions, 0 deletions